In such a TDMA system the messages to be sent out arrive concurrently over local channels at a concentrating terminal associated with the transmitting ground station for sampling, digital coding and arrangement in time slots temporarily allocated to respective channels in a recurrent outgoing PCM frame, normally of 125 .mu.s. Such a frame may have 32 time slots, two of them (e.g. the 1.sup.st and the 17.sup.th) being reserved for synchronizing and supervisory signals including address codes identifying the destinations of the messages conveyed in the remaining 30 time slots. The digitized message sample in each active time slot is normally an 8-bit byte, corresponding a sampling rate of 64 kbit/sec. The overall bit rate of such a PCM frame is 2Mbit/sec.
The characteristics of TDMA terminals present at the ground stations designed for satellite links are internationally specified by the most important space authorities, such as Intelsat and Eutelsat.
It is well known that at a TDMA ground station a 2Mbit/sec PCM stream is to undergo various operations before being utilized to modulate the phase of the radio-frequency carrier used for transmission to the satellite. More particularly, sixteen data words in the form of 8-bit samples or bytes of each channel can be grouped together in orderly sequence. Within a given time interval of 2 ms, corresponding to a standardized TDMA frame period, the 128 bits of such a 16-byte sequence are converted to a higher speed and combined with the bits of other PCM streams by a multiplexing operation, thus yielding a standard outgoing TDMA frame of the same duration as the initial sequence.
A succession of such TDMA frames, e.g. 16 as described in commonly owned application Ser. No. 368,297 filed by one of us (Gian Battista Alaria) jointly with another on Apr. 14, 1982, can be grouped into a superframe in which the last TDMA period is dedicated to a synchronization code and other supervisory signals.
In a system of the SS/TDMA (satellite-switched time-division multiple-access) type the relay station aboard the satellite carries out switching functions in response to instructions received from the ground. A switching unit of such a relay station has been described, for example, in commonly owned U.S. Pat. No. 4,215,348.
As far as we are aware, precise specifications relating to SS/TDMA installations have not yet been internationally defined and no standards have been established for the on-board switching unit or for the equipment controlling same on the ground. The art shows an increasing tendency toward a combination of time-division and space-division switching stages requiring temporary storage of an entire TDMA frame; this is due to the fact that such switching units assure greater system efficiency in terms of traffic to be handled by a given structure. Such a combined temporal/spatial switching unit has been disclosed, for example, in commonly owned U.S. Pat. No. 4,386,425.
The circuitry needed for the routing of traffic with interim frame storage through a switching unit of, say, the TST (temporal/spatial/temporal) type is rather complex and can therefore not be installed aboard a satellite, at least with current technology. Existing ground stations, on the other hand, require modifications in order to accommodate the components necessary for remote routing control. Such modification, moreover, should take into account the desirability of minimizing the length of a TDMA frame in order to simplify the on-board memories serving for interim data storage. Furthermore, in a system in which several ground stations share a common up-link and down-link, the routing must be properly timed to coincide with transmission and reception windows assigned to a given ground station.